The invention relates to a process for removing a polymeric covering from the surface of an optical fiber followed by applying a protective recoating composition after modifying the structure of the optical fiber. More particularly the present invention provides a process including physical and chemical steps to remove coating from a portion of a coated optical fiber while shaping transition regions of coating adjacent to the boundaries of a stripped portion of the optical fiber.
Interconnection of fiber optic networks requires high precision devices in the form of optical connectors that join optical fibers to peripheral equipment and other optical fibers while maintaining adequate signal strength. In operation an optical connector centers the small fiber so that the light gathering core lies directly over and in alignment with a light transmitting source or another fiber. Sections of optical fiber may also be spliced together using mechanical splicing or fusion splicing techniques.
Special features may be built into selected, relatively short lengths of optical fibers to be spliced into fiber optic networks. An optical fiber Bragg grating represents a light-modifying feature that may be introduced or written into an optical fiber by exposure to ultraviolet light. The ability to write such gratings leads to a variety of devices. Bragg gratings may be applied in telecommunications systems, for example, to control the wavelength of laser light, and to introduce dispersion compensation. Fiber optic applications of fiber Bragg gratings, outside of telecommunications, include spectroscopy and remote sensing.
The process of introducing special features such as Bragg gratings into an optical fiber may include a number of steps requiring handling of relatively short lengths of optical fiber during a series of manufacturing operations. An optical fiber typically requires removal of protective coatings before changing the physical characteristics of the fiber to include a Bragg grating. One manufacturing process requires the removal of protective buffers and coatings to reveal the bare surface of an optical fiber. Several processes are known for removing protective layers, such as buffers and coatings, from the surface of optical fibers. They include mechanical stripping, chemical stripping and thermal stripping.
Mechanical stripping of optical fibers and related coated filaments requires careful positioning of sharp tempered metal blades to expose a bare surface portion of a fiber without cutting or scratching or otherwise physically damaging the fiber surface. Known methods of mechanical stripping relate to cutting blade design and how a coating may be removed from the surface of a fiber. The predominant use of mechanical stripping involves the removal of protective layers from the ends of optical fibers, insulated wires and related filaments, prior to joining the filament ends together. U.S. Pat. No. 4,434,554 describes an optical fiber stripping device including a flat base having a number of fiber receiving channels of suitable depth to ensure only removal of a buffer coating from each fiber, when a blade penetrates the coating. The blade moves parallel to the axis of a fiber or group of fibers using a paring action to remove protective material. Channel size, based upon fiber diameter determines the selection of a flat base to provide a device that strips a fiber end without damaging the fiber itself.
One way to avoid damage to the bare surface of an optical fiber requires the use of blades designed to penetrate the protective buffer or fiber coating without reaching the fiber surface. Suitable blades have a substantially semicircular sharpened edge of a radius slightly larger than the radius of the bare optical fiber. Two opposing blades, penetrating the protective layer around the fiber, interfere with each other before the cutting edges reach the fiber surface. After penetrating a protective layer, close to the end of a fiber, movement of the blades parallel to the fiber axis displaces a section of the layer to provide a bare fiber end untouched by the blades.
Japanese patent JP 875930 uses a mechanical stripping process to remove coating from a section of optical fiber. Initially, an angled cutting blade rotates about two separated points to form notches in the circumference of the buffer coating over the optical fiber. A separate straightedge blade then moves parallel to the fiber axis to remove coating material from between the sharply angled notches.
U.S. Pat. Nos. 4,630,406, 5,269,206, 5,481,638, 5,684,910, and 5,819,602 describe the manufacture and design of blades for cutting insulation from e.g. insulated electrical wires and optical fibers. Successful mechanical stripping using such blades may require additional treatments, including softening the protective layer as in U.S. Pat. No. 5,481,638 requiring a chemical filled chamber first to soften an encapsulating layer then to clean plastic material from the blades after stripping. U.S. Pat. No. 5,684,910 teaches an optical fiber having improved mechanical strippability. The improvement includes the use of a frangible boundary layer between a fiber coating and a buffer to facilitate separation from the bare fiber. Previous teachings include initial blade movement perpendicular to a filament axis, to penetrate a coating, followed by movement parallel to the filament axis to expose bare filament ends by displacement of protective layers.
Chemical stripping may be used as an alternative to mechanical stripping for preparing bare fiber ends. U.S. Pat. Nos. 4,865,411 and 4,976,596 deal with controlled removal of coating, by gradual withdrawal of a coated fiber from a chemical bath, to produce a contoured shallow taper adjacent to the bare glass fiber surface. A fixture, according to U.S. Pat. No. 5,451,294 provides support while dipping the end of a coated optical fiber into a chemical bath to dissolve coating from the end. Organic solvents and related softening agents may be used to remove coatings from optical fibers as described in U.S. Pat. Nos. 5,567,219, 5,681,417, 5,714,196, and 5,896,787. Chemical stripping methods include common problems related to the handling of chemicals especially, when the chemical strippers involve corrosive liquids.
Stripping by rapid heating may be used instead of mechanical or chemical stripping. One example of this process, described in U.S. Pat. No. 6,123,801, uses a hot inert gas to melt buffer coating and blow it from the surface of an optical fiber. The process requires high pressure gas streams and temperatures in the region of 800xc2x0 C. to strip coating from the fiber. U.S. Pat. No. 5,939,136 describes a process for preparing optical fiber devices including thermal removal of a coating from an optical fiber, preferably performed using a heated gaseous stream. U.S. Pat. Nos. 5,964,957 and 5,968,283 further describe the use of heat to remove coatings from optical fibers.
A reason for removing protective buffers and related coatings from an inner section of optical fibers is the need to change the characteristics of the fiber such as by writing of a refractive index grating, also known as a Bragg grating, in the core of an optical fiber. Refractive index changes occur during exposure of a bare fiber to radiation from an ultraviolet laser or similar exposure device. The majority of protective coatings for optical fibers absorb the fiber modifying radiation. This explains the need to remove the coatings before writing a refractive index grating. Fibers coated to a thickness exceeding 400 xcexcm and those having silicone containing coatings respond poorly to mechanical stripping and chemical stripping as methods for removing optical fiber coatings.
Chemical stripping using hot concentrated sulfuric acid does not always displace optical fiber coatings as expected. Thick, silicone-containing coatings, in particular, may react poorly in the presence of hot sulfuric acid. Some coatings may not dissolve cleanly, or may tend to form gelatinous strands that adhere to the optical fiber leaving discolored, or charred material that is difficult to remove from the surface of a bare fiber. This is particularly undesirable if gel material remains attached to sections of optical fiber from which coating should be stripped. Partially dissolved coatings may also detach from a fiber and contaminate other fibers or the bath. Chemical contamination may also result from acid stripping of silicone-containing coatings.
Problems may also be encountered with mechanical stripping of some coated optical fibers. A thick outer coating is tough and difficult to remove with conventional mechanical stripping devices. A mechanical stripping method cannot normally displace all of the coating from an optical fiber to provide a clean, bare optical fiber surface. For this reason it is conventional to use a combination of mechanical stripping and chemical or thermal stripping to remove the coating from a coated optical fiber to expose a clean bare surface.
An optical fiber including a refractive index grating also has a bare portion, after stripping, that requires application of protective coatings before becoming part of an optical fiber device. The widely accepted method for recoating bare sections of optical fibers involves special coating molds. Methods similar to those used to coat drawn fibers, during their manufacture have also been described.
A recoating mold, described in U.S. Pat. No. 4,410,561, provides a coated optical fiber using a split mold die structure. The size and design of a cavity formed by the closed mold provides space that becomes filled during injection of curable, protective, fluid recoating compositions. It is desirable to avoid entrapment of air inside the mold since this could lead to a defective recoated fiber section. Complete filling of a mold cavity may involve intentional application of pressure. U.S. Pat. No. 5,022,735 uses a screw type plunger to pressurize recoating fluid injected into a conventional recoating mold. Some recoating molds include curing means to provide finished recoated sections of optical fibers. U.S. Pat. No. 4,662,307, for example, uses a split mold including an injection port and UV light port through which light passes to cure recoating compositions. The curing process requires multiple light sources.
Conventional stripping to remove coating from a section of an optical fiber addresses how to remove the coating to obtain a, clean and bare length of optical fiber between spaced-apart boundaries that have bare fiber on one side and original coating or buffer on the other. A coating over an optical fiber may include a primary buffer and a secondary buffer. Short distances on either side of each boundary may be referred to as transition regions of a stripped optical fiber. In most cases, the coating at each boundary is disposed at a sharp angle to a bare fiber. In some cases, e.g. U.S. Pat. Nos. 4,865,411, 4,976,596 and 5,451,294 the transition regions may have a conical shape. However these references use chemical shaping that is time consuming and difficult to control, and may not be appropriate for all fibers.
Transition regions represent points of weakness of a stripped section of a coated fiber, especially when the fiber is a brittle, glass optical fiber. A previously stripped section of fiber may be strengthened somewhat by application of a recoating material that protects and, to a certain extent, provides some support to the fragile glass fiber. However, even a recoated fiber may exhibit weakness in its transition regions, especially when the severed boundaries of the coatings make a sharp angle, e.g. a 90xc2x0 angle, with the fiber axis. Weakness may appear as cracking or breaking of the recoating material in the transition regions after long term use or accelerated aging that occurs by thermal recycling of recoated fibers, particularly optical fibers. At times there is enough crack propagation to cause the formation of a gap between a recoating material and the original fiber coating. This occurs because of stress concentration within the transition regions and particularly at the boundary between the original coating and the recoating material.
The use of a high Young""s modulus resin as the recoating material offers one approach for preventing crack propagation within transition regions of recoated fibers. This solution is not fully effective because resins having a high Young""s modulus may adversely impact the performance of a refractive index grating contained by the recoated section of optical fiber, compared to a conventional recoating resin material.
Some fibers with a hard polymer coating material require pre-treatment of the coated fiber to soften the original polymer coating. Pretreatment can adversely affect the rate and yield of recoated fibers.
Each point in the processes, of fiber stripping, modifying, e.g. to introduce a refractive index grating, and recoating, requires care to prevent damaging the fragile optical fiber. Damage to optical fibers may occur by physical contact or exposure to tensile, torsional, twisting, and bending stresses. Excessive bending can change the optical characteristics of a fiber. Failure to meet required optical characteristics leads to rejection of an optical device and increases the expense of device manufacture. A need exists for improved means for stripping and recoating processes to reduce incidence of damage, thereby reducing the cost and increasing the yield of optical fiber devices. The resulting, recoated fiber should be highly reliable.
The present invention provides a process for preparing and stripping a central portion of a coated fiber to provide a shaped transition region adjacent to the bare surface of the stripped fiber. Thereafter, the contours of the shaped transition region promote improved bonding with recoating compositions later applied to cover transition regions and the stripped central portion of a fiber. A preferred method according to the present invention includes the steps of boundary cutting, transition region shaping, solvent treatment and coating displacement.
Coated fibers suitable for use with the present invention include elongate filaments having a central core overcoated with at least one layer of protective material. Examples of such filaments include insulated wires and particularly optical fibers. A variety of devices use optical fibers that require structural modification to include one or more in-line optical waveguide refractive index gratings, also known as Bragg gratings. At least one Bragg grating may be formed in at least a portion of the length of an optical fiber. Formation of an optical fiber Bragg grating typically requires a series of operations including mechanical stripping of protective coatings, and chemical stripping to expose the fiber by removing residual material down to the clean optical fiber surface. The writing of Bragg gratings into optical fibers is well known using patterns of ultraviolet radiation to alter optical fiber index of refraction characteristics. An optical fiber that contains a Bragg grating in a stripped section is susceptible to damage by chemical attack or by physical contact and exposure to tensile, torsional, twisting, and bending stresses. Excessive bending can change the optical characteristics of a fiber. Failure to meet required optical characteristics leads to rejection of an optical device. Recoating the previously stripped section of optical fiber reduces susceptibility to damage. Testing by thermal recycling and visual inspection confirms attainment of performance requirements desired of a recoated optical fiber Bragg grating.
A process according to the present invention for stripping coating from a filament, particularly an optical fiber, down to a cleaned fiber surface begins by establishing the boundaries of a section of fiber for removal of coating. Formation of cut boundaries requires radial cutting through most of or all of the thicker, outer, secondary buffer using one or more blades, preferably a pair of opposed blades operating at right angles to the axis of the fiber. Incisions through the secondary buffer may extend into but not through the primary buffer that is closest to the core structure of the fiber. Equipment used to form cut boundaries preferably includes suitably positioned stops to prevent incisions from reaching too far into the coating covering the bare fiber. The cut boundaries represent a discontinuity in the secondary buffer over the fiber. Preferably, the equipment used to form cut boundaries includes a pair of blades that simultaneously produce incisions at each of the cut boundaries. During this process the pairs of blades have a separation corresponding to the length of the section of fiber to be stripped of original coating material. The distance separating the pairs of blades is preferably about 2.54 cm (1 inch). Depending on the nature of the process steps following optical fiber stripping, the length of coating stripped from a coated optical fiber may range from about 1 cm to about 4.5 cms, preferably between about 1.5 cms and 3.6 cms.
After formation of the cut boundaries and withdrawal of the blades from the incisions, an increased separation of the blades moves them outside the locations of the cut boundaries. Optionally the coated optical fiber may be transferred to a piece of equipment designed for shaping transition regions from points outside the cut boundaries. The shaping process may use either a single skiving blade or a pair of angled, straight-edged skiving blades or a device to abrade the transition region to a desired shape. Using skiving blades at a fixed blade angle, a single pass or multiple passes along the fiber section introduces opposing shoulders angling towards flattened steps on one side or opposing sides of the fiber. The latter, preferred case corresponds to the placement of skiving blades above and below the fiber or in a similar opposing relationship. The flattened steps lie outside of the cut boundaries to form parts of transition regions after the removal of coating from between the cut boundaries. It will be appreciated that removal of coating may involve one or more sections around the circumference of a coated optical fiber, depending upon the desired shape of the transition region. After shaping both sides of the coating, the thickness remaining on either side of the optical fiber is the same between the cut boundaries as it is between the flattened step of each transition region. Also, between the cut boundaries the depth of penetration into the secondary buffer exposes a length of primary buffer for at least the distance between the cut boundaries. Upon completion of the shaping process, preferably the secondary buffer is discontinuous at each of the cut boundaries as described previously. In addition, exposure of the primary buffer causes a gap on opposing sides of the secondary buffer. Removal of the underlying primary buffer should release the secondary buffer from around the section of fiber to be stripped.
The stripping process according to the present invention applies particularly to coated fibers including solvent swellable primary buffers. After completion of the shaping process, immersion of the affected fiber section in a suitable solvent causes swelling of the primary buffer or coating preferably between the cut boundaries. Swelling of the primary buffer also weakens the connection between the secondary buffer and the fiber. In this condition the secondary buffer may be relatively easily displaced from the surface of the fiber, either by picking or otherwise gently dislodging the coating from the fiber. One method for gently dislodging insoluble coating from a fiber involves the use of at least a pair of resilient wiping blades or pads drawn between one cut boundary and the other for effective removal of residual coating loosened by swelling of the primary buffer. Removal of the loosened secondary buffer preferably uses a deformable material that displaces secondary buffer without damaging the fiber. Particularly tenacious coatings may require treatment with acid as a final cleaning step.
After removing its protective coating and applying tension to the stripped section of an optical fiber, the Bragg grating writing process proceeds preferably while observing a spectrum analyzer display of the wavelength envelope produced by the writing process. This provides feedback of the quality of a grating at the time of writing and represents a convenient decision point for acceptance or rejection a fiber Bragg grating as it is written.
Application of recoating material to protect a Bragg grating formed in an optical fiber represents the final step according to the recoating process of the present invention. A final check of the resulting product determines if it passes visual inspection requirements and proof testing to measure tensile strength. A stripped optical fiber containing a Bragg grating may be recoated using a conventional split recoating mold that has two sections each with a semicircular groove formed in its surface. After closing the mold a cylindrical cavity forms around the fiber portion that needs recoating. Fluid injected through an opening in the mold surrounds the stripped fiber portion before curing to provide the recoated optical fiber. Commercially available recoating compositions may be used for this purpose.
More particularly the present invention provides a process for recoating a stripped optical fiber comprising the steps of, providing an optical fiber having a coating and cutting a first cut boundary spaced from a secondary cut boundary to mark an internal section of the coating, the internal section having opposing sides. Removal of the coating from at least one of the opposing sides provides a pared intervening layer and further forms a first transition region opposite a second transition region. Each of the first transition region and the second transition region has a substantially wedge-shaped contour. Solvent treatment of the pared intervening layer weakens the bond between the coating and the optical fiber between the first cut boundary and the second cut boundary before displacement of the coating from the optical fiber to provide the stripped optical fiber. The stripped optical fiber includes a bare section of optical fiber, the first transition region and the second transition region. After applying recoating material, the stripped optical fiber becomes the recoated optical fiber in which the recoating material covers the bare section of optical fiber, the first transition region and the second transition region.
The invention further includes a recoated optical fiber comprising a first transition region having a substantially wedge-shaped contour adjacent to a first cut boundary. A second transition region also having a substantially wedge-shaped contour lies opposite the first transition region, and adjacent to a second cut boundary. The recoated optical fiber further includes a section of optical fiber, between the first cut boundary and the second cut boundary, and a recoating material having a bond to the first transition region, the section of optical fiber and the second transition region.
Definitions
The terms xe2x80x9cbare fiber,xe2x80x9d or xe2x80x9cbare fiber portion,xe2x80x9d or xe2x80x9cstripped fiber,xe2x80x9d or phrases relating to such terms refer herein to the portion of an optical fiber from which protective coating has been removed to expose the silica surface of the fiber.
The terms xe2x80x9cbufferxe2x80x9d or xe2x80x9cprimary bufferxe2x80x9d or xe2x80x9cprimary coatingxe2x80x9d refer herein to a polymer or resin layer next to a bare fiber.
A xe2x80x9csecondary coatingxe2x80x9d or xe2x80x9csecondary bufferxe2x80x9d is used herein to describe a polymer or resin layer next to a buffer or primary buffer.
The term xe2x80x9cresinxe2x80x9d as used herein is a general term describing polymer coverings for filaments particularly optical fibers. Materials used for previously defined buffers and coatings fall within the general term of resin.
The term xe2x80x9cfilamentxe2x80x9d herein refers to a fiber structure, preferably a xe2x80x9csilica filament.xe2x80x9d An optical fiber is a preferred form of filament according to the present invention.
A xe2x80x9ctransition regionxe2x80x9d describes the preferably quasi-wedge shape of the portion of buffer layers closest to a bare fiber portion after subjecting a coated optical fiber to stripping according to the present invention.
The term xe2x80x9ccut boundaryxe2x80x9d refers to an incision made in the circumference of a coated optical fiber to substantially penetrate a secondary buffer coating with minimal penetration of a primary buffer coating. A separation between a first cut boundary and a second cut boundary represents the section of coating to be stripped from the surface of the bare fiber.
The term xe2x80x9cpared intervening layerxe2x80x9d is used herein to describe a structure formed by removing coating material from around an optical fiber. In this case paring includes removal of material using either sharpened blades or some form of abrasive.
The present invention has been developed to provide a process and equipment for conveniently handling a filament in the form of an optical fiber during multiple processing operations that may be at least partially automated as a further benefit to the user. These enhancements and benefits are described in greater detail hereinbelow with respect to the several aspects and alternative embodiments of the present invention.